PROJECT SUMMARY Immune memory is defined as the capacity of a host to respond more vigorously to a secondary antigen encounter. Such accelerated anamnestic response is the basis for vaccination, and understanding the factors that regulate immune memory is essential for the rational design of vaccines. On the other hand, immune exhaustion is caused by chronic viral infections and cancers, and such process is accompanied by impaired immunity to cognate and unrelated antigens. T regulatory cells (Tregs) have been shown to distinctively regulate immune memory and immune exhaustion, but their mechanisms remain poorly explored. Prior data have demonstrated that Tregs suppress self-reactive T cell responses, as well as exhausted CD8 T cell responses during chronic infections and cancers, consistent with their well-defined suppressive role. However, we and others have recently shown that Tregs promote memory CD8 T cell responses during acutely controlled infection or vaccination, suggesting that Tregs regulate CD8 T cells differently. It is important to understand how CD8 T cells are regulated by Tregs because CD8 T cells are critical for the control of chronic infections and cancers, which cause considerable morbidity and mortality. Our preliminary data suggest that IFN, CXCL10 and mTOR may determine the effects of Tregs on memory and exhausted CD8 T cells. Altogether, our proposal will address two Specific Aims: Aim 1. To determine if Tregs regulate memory and exhausted CD8 T cells via IFN, CXCL10 and mTOR. This Aim will explore how Tregs regulate CD8 T cell differentiation following acute or chronic viral infection. We will also investigate the mechanism of how Tregs develop hyper-suppressive functions during chronic infection. Aim 2. To evaluate if therapeutic modulation of IFN, CXCL10 and mTOR can revert CD8 T cell exhaustion during chronic infection. We aim to develop highly effective immunotherapies to rescue CD8 T cell responses considering not only PD-1, a CD8 T cell-intrinsic immune checkpoint, but also Treg-intrinsic pathways that show profound enrichment during chronic infection, and which have been proposed to play inhibitory roles. Overall, these experiments will be important for improving our basic understanding of the immune system, for rational vaccine design, and for developing therapies to revert immune exhaustion during antigen persistence.